Acta Cryst. (2007). E63, i157 [ doi:10.1107/S1600536807027043 ]
Single crystals of dirubidium dicalcium tris[sulfate(VI)], Rb2Ca2(SO4)3, were obtained from solid-state reactions of Rb2SO4 and CaSO4. The title compound crystallizes in the cubic langbeinite-type structure. It features two crystallographically independent CaO6 octahedra (each with site symmetry 3), which are linked by sharing corners with SO4 tetrahedra to establish a framework with composition [Ca2(SO4)3]2-, where the two independent Rb+ cations (site symmetry 3) are located in the voids.
Single crystals of Rb2Ca2(SO4)3 were obtained by solid-state reaction of Rb2SO4 (Aldrich 99.999%) and CaSO4.H2O (Aldrich 99.9%). Stoichiometric amounts of the starting materials were mixed thoroughly in an agate mortar. After grinding, the mixture was heated at 673 K for 4 h, then at 1173 K for 66 h and was finally allowed to cool to room temperature at a rate of 5 K/h. Transparent polycrystalline chunks were obtained from which single crystals were separated manually.
Data collection: STADI4 (Stoe & Cie, 2000); cell refinement: STADI4; data reduction: X-RED (Stoe & Cie, 1996); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molecular graphics: DIAMOND (Brandenburg & Berndt, 1999); software used to prepare material for publication: CRYSTALS.
![[Figure 1]](./wm2116fig1thm.gif)
| Fig. 1. Projection of the crystal structure of langbeinite-type Rb2Ca2(SO4)3 approximately along [001]. The voids are visible, where the Rb+ cations (lavender spheres) are located. CaO6 octahedra are dark-grey and SO4 tetrahedra are light-grey. |
| Rb2Ca2(SO4)3 | Z = 4 |
| Mr = 539.28 | F000 = 1032 |
| Cubic, P213 | Dx = 3.048 Mg m−3 |
| Hall symbol: P 2ac 2ab 3 | Mo Kα radiation λ = 0.71073 Å |
| a = 10.553 (3) Å | Cell parameters from 23 reflections |
| b = 10.553 (3) Å | θ = 2.7–30.1º |
| c = 10.553 (3) Å | µ = 9.79 mm−1 |
| α = 90º | T = 293 K |
| β = 90º | Fragment, colourless |
| γ = 90º | 0.22 × 0.13 × 0.05 mm |
| V = 1175.2 (6) Å3 |
| Stoe–Siemens AED2 four-circle diffractometer | Rint = 0.08 |
| Monochromator: graphite | θmax = 29.9º |
| T = 298 K | θmin = 2.7º |
| ω/2θ scans | h = −8→9 |
| Absorption correction: multi-scan (MULABS in PLATON; Spek, 2003) | k = −10→10 |
| Tmin = 0.231, Tmax = 0.644 | l = −14→14 |
| 1236 measured reflections | 2 standard reflections |
| 1080 independent reflections | every 30 min |
| 658 reflections with I > 3σ(I) | intensity decay: 0.7% |
| Refinement on F2 | w = 1/[σ2(Fo2) + (0.021P)2 + 6.5703P] where P = (Fo2 + 2Fc2)/3 Method, part 1, Chebychev polynomial, (Watkin (1994). Acta Cryst. A50, 411–437. Prince (1982) Mathematical Techniques in Crystallography and Materials Science. New York: Springer-Verlag.] [weight] = 1.0/[A0*T0(x) + A1*T1(x) ··· + An-1]*Tn-1(x)] where Ai are the Chebychev coefficients listed below and x = F /Fmax Method = Robust Weighting (Prince, 1982) W = [weight] * [1-(deltaF/6*sigmaF)2]2 Ai are: 2.38 -2.95 2.50 -1.02 0.340 |
| Least-squares matrix: full | (Δ/σ)max = 0.001 |
| R[F2 > 2σ(F2)] = 0.065 | Δρmax = 0.93 e Å−3 |
| wR(F2) = 0.128 | Δρmin = −1.00 e Å−3 |
| S = 1.12 | Extinction correction: SHELXL |
| 1080 reflections | Extinction coefficient: 0.0008 (5) |
| 59 parameters | Absolute structure: Flack (1983), 558 Friedel pairs |
| Primary atom site location: structure-invariant direct methods | Flack parameter: 0.00 (4) |
| Rb2Ca2(SO4)3 | γ = 90º |
| Mr = 539.28 | V = 1175.2 (6) Å3 |
| Cubic, P213 | Z = 4 |
| a = 10.553 (3) Å | Mo Kα |
| b = 10.553 (3) Å | µ = 9.79 mm−1 |
| c = 10.553 (3) Å | T = 293 K |
| α = 90º | 0.22 × 0.13 × 0.05 mm |
| β = 90º |
| Stoe–Siemens AED2 four-circle diffractometer | 658 reflections with I > 3σ(I) |
| Absorption correction: multi-scan (MULABS in PLATON; Spek, 2003) | Rint = 0.08 |
| Tmin = 0.231, Tmax = 0.644 | 2 standard reflections |
| 1236 measured reflections | every 30 min |
| 1080 independent reflections | intensity decay: 0.7% |
| R[F2 > 2σ(F2)] = 0.065 | Δρmax = 0.93 e Å−3 |
| wR(F2) = 0.128 | Δρmin = −1.00 e Å−3 |
| S = 1.12 | Absolute structure: Flack (1983), 558 Friedel pairs |
| 1080 reflections | Flack parameter: 0.00 (4) |
| 59 parameters |
| x | y | z | Uiso*/Ueq | ||
| Ca1 | 0.3314 (2) | 0.3314 (2) | 0.3314 (2) | 0.0170 (9) | |
| Ca2 | 0.5920 (2) | 0.5920 (2) | 0.5920 (2) | 0.0170 (9) | |
| Rb1 | 0.81628 (13) | 0.81628 (13) | 0.81628 (13) | 0.0270 (5) | |
| Rb2 | 0.04991 (13) | 0.04991 (13) | 0.04991 (13) | 0.0291 (6) | |
| S | 0.2243 (3) | 0.3749 (3) | 0.0108 (3) | 0.0136 (6) | |
| O1 | 0.3089 (9) | 0.2795 (8) | 0.9600 (9) | 0.032 (2) | |
| O2 | 0.0954 (8) | 0.3285 (10) | 0.0046 (10) | 0.039 (3) | |
| O3 | 0.2364 (10) | 0.4880 (10) | 0.9326 (11) | 0.043 (3) | |
| O4 | 0.2580 (10) | 0.4059 (11) | 0.1420 (9) | 0.044 (3) |
| U11 | U22 | U33 | U12 | U13 | U23 | |
| Ca1 | 0.0170 (9) | 0.0170 (9) | 0.0170 (9) | −0.0002 (10) | −0.0002 (10) | −0.0002 (10) |
| Ca2 | 0.0170 (9) | 0.0170 (9) | 0.0170 (9) | −0.0004 (10) | −0.0004 (10) | −0.0004 (10) |
| Rb1 | 0.0270 (5) | 0.0270 (5) | 0.0270 (5) | −0.0023 (6) | −0.0023 (6) | −0.0023 (6) |
| Rb2 | 0.0291 (6) | 0.0291 (6) | 0.0291 (6) | 0.0012 (6) | 0.0012 (6) | 0.0012 (6) |
| S | 0.0156 (15) | 0.0120 (13) | 0.0131 (15) | 0.0031 (11) | 0.0003 (11) | 0.0015 (10) |
| O1 | 0.034 (5) | 0.023 (5) | 0.039 (6) | 0.011 (4) | 0.012 (5) | −0.006 (4) |
| O2 | 0.015 (5) | 0.037 (6) | 0.064 (7) | −0.007 (4) | 0.011 (5) | −0.016 (6) |
| O3 | 0.059 (7) | 0.030 (6) | 0.041 (6) | 0.015 (5) | 0.008 (5) | 0.023 (5) |
| O4 | 0.060 (8) | 0.047 (7) | 0.026 (6) | −0.003 (6) | −0.009 (5) | −0.011 (5) |
| Ca1—O4i | 2.284 (10) | Rb1—O3xxiii | 3.464 (13) |
| Ca1—O4ii | 2.284 (10) | Rb1—O3xxiv | 3.464 (13) |
| Ca1—O4 | 2.284 (10) | Rb2—O2i | 3.018 (10) |
| Ca1—O3iii | 2.299 (10) | Rb2—O2 | 3.018 (10) |
| Ca1—O3iv | 2.299 (10) | Rb2—O2ii | 3.018 (10) |
| Ca1—O3v | 2.299 (10) | Rb2—O1xxv | 3.118 (10) |
| Ca1—Rb1vi | 4.0344 (18) | Rb2—O1xxvi | 3.118 (10) |
| Ca1—Rb1vii | 4.0344 (18) | Rb2—O1xxvii | 3.118 (10) |
| Ca1—Rb1viii | 4.0344 (18) | Rb2—O3xxvi | 3.338 (11) |
| Ca1—Rb2ix | 4.804 (2) | Rb2—O3xxvii | 3.338 (11) |
| Ca1—Rb2x | 4.804 (2) | Rb2—O3xxv | 3.338 (11) |
| Ca1—Rb2xi | 4.804 (2) | Rb2—Sxxviii | 3.584 (3) |
| Ca2—O1xii | 2.304 (9) | Rb2—Sxxix | 3.584 (3) |
| Ca2—O1xiii | 2.304 (9) | Rb2—Sxxx | 3.584 (3) |
| Ca2—O1xiv | 2.304 (9) | S—O1xxxi | 1.449 (9) |
| Ca2—O2xv | 2.338 (9) | S—O2 | 1.447 (9) |
| Ca2—O2xvi | 2.338 (9) | S—O3xxxi | 1.456 (10) |
| Ca2—O2xvii | 2.338 (9) | S—O4 | 1.466 (10) |
| Ca2—Sxv | 3.464 (4) | S—Ca2xxvii | 3.464 (4) |
| Ca2—Sxvi | 3.464 (4) | S—Rb1v | 3.533 (3) |
| Ca2—Sxvii | 3.464 (4) | S—Rb2xi | 3.584 (3) |
| Ca2—Rb2xv | 4.0891 (19) | S—Rb1viii | 3.859 (3) |
| Ca2—Rb2xviii | 4.0891 (19) | O1—Sxxxii | 1.449 (9) |
| Ca2—Rb2xvii | 4.0891 (19) | O1—Ca2vii | 2.304 (9) |
| Rb1—O4xix | 3.028 (11) | O1—Rb2xv | 3.118 (10) |
| Rb1—O4xx | 3.028 (11) | O1—Rb1vii | 3.224 (9) |
| Rb1—O4xxi | 3.028 (11) | O2—Ca2xxvii | 2.338 (9) |
| Rb1—O1xii | 3.224 (9) | O2—Rb1v | 3.515 (10) |
| Rb1—O1xiii | 3.224 (9) | O3—Sxxxii | 1.456 (10) |
| Rb1—O1xiv | 3.224 (10) | O3—Ca1xviii | 2.299 (10) |
| Rb1—O3xiv | 3.239 (12) | O3—Rb1vii | 3.239 (12) |
| Rb1—O3xii | 3.239 (12) | O3—Rb2xv | 3.338 (11) |
| Rb1—O3xiii | 3.239 (12) | O3—Rb1xxxiii | 3.464 (13) |
| Rb1—O3xxii | 3.464 (13) | O4—Rb1viii | 3.028 (11) |
| O4i—Ca1—O4ii | 96.9 (4) | O3xii—Rb1—O3xxiii | 138.68 (19) |
| O4i—Ca1—O4 | 96.9 (4) | O3xiii—Rb1—O3xxiii | 58.5 (4) |
| O4ii—Ca1—O4 | 96.9 (4) | O3xxii—Rb1—O3xxiii | 82.8 (3) |
| O4i—Ca1—O3iii | 90.9 (4) | O4xix—Rb1—O3xxiv | 107.7 (3) |
| O4ii—Ca1—O3iii | 81.3 (4) | O4xx—Rb1—O3xxiv | 54.3 (3) |
| O4—Ca1—O3iii | 172.1 (4) | O4xxi—Rb1—O3xxiv | 42.4 (2) |
| O4i—Ca1—O3iv | 81.3 (4) | O1xii—Rb1—O3xxiv | 96.7 (2) |
| O4ii—Ca1—O3iv | 172.1 (4) | O1xiii—Rb1—O3xxiv | 131.6 (2) |
| O4—Ca1—O3iv | 90.9 (4) | O1xiv—Rb1—O3xxiv | 145.4 (2) |
| O3iii—Ca1—O3iv | 91.0 (4) | O3xiv—Rb1—O3xxiv | 138.68 (19) |
| O4i—Ca1—O3v | 172.1 (4) | O3xii—Rb1—O3xxiv | 58.5 (4) |
| O4ii—Ca1—O3v | 90.9 (4) | O3xiii—Rb1—O3xxiv | 104.18 (3) |
| O4—Ca1—O3v | 81.3 (4) | O3xxii—Rb1—O3xxiv | 82.8 (3) |
| O3iii—Ca1—O3v | 91.0 (4) | O3xxiii—Rb1—O3xxiv | 82.8 (3) |
| O3iv—Ca1—O3v | 91.0 (4) | O2i—Rb2—O2 | 91.4 (3) |
| O1xii—Ca2—O1xiii | 85.6 (4) | O2i—Rb2—O2ii | 91.4 (3) |
| O1xii—Ca2—O1xiv | 85.6 (4) | O2—Rb2—O2ii | 91.4 (3) |
| O1xiii—Ca2—O1xiv | 85.6 (4) | O2i—Rb2—O1xxv | 64.0 (2) |
| O1xii—Ca2—O2xv | 172.2 (4) | O2—Rb2—O1xxv | 153.1 (3) |
| O1xiii—Ca2—O2xv | 88.5 (3) | O2ii—Rb2—O1xxv | 79.1 (2) |
| O1xiv—Ca2—O2xv | 89.0 (3) | O2i—Rb2—O1xxvi | 79.1 (2) |
| O1xii—Ca2—O2xvi | 89.0 (3) | O2—Rb2—O1xxvi | 64.0 (2) |
| O1xiii—Ca2—O2xvi | 172.2 (4) | O2ii—Rb2—O1xxvi | 153.1 (3) |
| O1xiv—Ca2—O2xvi | 88.5 (3) | O1xxv—Rb2—O1xxvi | 117.59 (9) |
| O2xv—Ca2—O2xvi | 96.4 (3) | O2i—Rb2—O1xxvii | 153.1 (3) |
| O1xii—Ca2—O2xvii | 88.5 (3) | O2—Rb2—O1xxvii | 79.1 (2) |
| O1xiii—Ca2—O2xvii | 89.0 (3) | O2ii—Rb2—O1xxvii | 64.0 (2) |
| O1xiv—Ca2—O2xvii | 172.2 (4) | O1xxv—Rb2—O1xxvii | 117.59 (9) |
| O2xv—Ca2—O2xvii | 96.4 (3) | O1xxvi—Rb2—O1xxvii | 117.59 (9) |
| O2xvi—Ca2—O2xvii | 96.4 (3) | O2i—Rb2—O3xxvi | 78.9 (3) |
| O4xix—Rb1—O4xx | 94.9 (3) | O2—Rb2—O3xxvi | 106.5 (2) |
| O4xix—Rb1—O4xxi | 94.9 (3) | O2ii—Rb2—O3xxvi | 159.8 (2) |
| O4xx—Rb1—O4xxi | 94.9 (3) | O1xxv—Rb2—O3xxvi | 80.6 (2) |
| O4xix—Rb1—O1xii | 155.6 (3) | O1xxvi—Rb2—O3xxvi | 42.5 (2) |
| O4xx—Rb1—O1xii | 99.5 (2) | O1xxvii—Rb2—O3xxvi | 127.9 (3) |
| O4xxi—Rb1—O1xii | 103.3 (3) | O2i—Rb2—O3xxvii | 159.8 (2) |
| O4xix—Rb1—O1xiii | 103.3 (3) | O2—Rb2—O3xxvii | 78.9 (3) |
| O4xx—Rb1—O1xiii | 155.6 (3) | O2ii—Rb2—O3xxvii | 106.5 (2) |
| O4xxi—Rb1—O1xiii | 99.5 (2) | O1xxv—Rb2—O3xxvii | 127.9 (3) |
| O1xii—Rb1—O1xiii | 58.1 (3) | O1xxvi—Rb2—O3xxvii | 80.6 (2) |
| O4xix—Rb1—O1xiv | 99.5 (2) | O1xxvii—Rb2—O3xxvii | 42.5 (2) |
| O4xx—Rb1—O1xiv | 103.3 (3) | O3xxvi—Rb2—O3xxvii | 86.7 (3) |
| O4xxi—Rb1—O1xiv | 155.6 (3) | O2i—Rb2—O3xxv | 106.5 (2) |
| O1xii—Rb1—O1xiv | 58.1 (3) | O2—Rb2—O3xxv | 159.8 (2) |
| O1xiii—Rb1—O1xiv | 58.1 (3) | O2ii—Rb2—O3xxv | 78.9 (3) |
| O4xix—Rb1—O3xiv | 62.7 (3) | O1xxv—Rb2—O3xxv | 42.5 (2) |
| O4xx—Rb1—O3xiv | 85.4 (3) | O1xxvi—Rb2—O3xxv | 127.9 (3) |
| O4xxi—Rb1—O3xiv | 157.5 (3) | O1xxvii—Rb2—O3xxv | 80.6 (2) |
| O1xii—Rb1—O3xiv | 98.8 (3) | O3xxvi—Rb2—O3xxv | 86.7 (3) |
| O1xiii—Rb1—O3xiv | 88.6 (2) | O3xxvii—Rb2—O3xxv | 86.7 (3) |
| O1xiv—Rb1—O3xiv | 42.6 (2) | O1xxxi—S—O2 | 109.1 (6) |
| O4xix—Rb1—O3xii | 157.5 (3) | O1xxxi—S—O3xxxi | 107.8 (6) |
| O4xx—Rb1—O3xii | 62.7 (3) | O2—S—O3xxxi | 109.5 (7) |
| O4xxi—Rb1—O3xii | 85.4 (3) | O1xxxi—S—O4 | 110.8 (6) |
| O1xii—Rb1—O3xii | 42.6 (2) | O2—S—O4 | 110.2 (6) |
| O1xiii—Rb1—O3xii | 98.8 (2) | O3xxxi—S—O4 | 109.3 (7) |
| O1xiv—Rb1—O3xii | 88.6 (2) | Sxxxii—O1—Ca2vii | 164.7 (6) |
| O3xiv—Rb1—O3xii | 114.23 (15) | Sxxxii—O1—Rb2xv | 96.5 (4) |
| O4xix—Rb1—O3xiii | 85.4 (3) | Ca2vii—O1—Rb2xv | 96.8 (3) |
| O4xx—Rb1—O3xiii | 157.5 (3) | Sxxxii—O1—Rb1vii | 89.9 (4) |
| O4xxi—Rb1—O3xiii | 62.7 (3) | Ca2vii—O1—Rb1vii | 94.3 (3) |
| O1xii—Rb1—O3xiii | 88.6 (2) | Rb2xv—O1—Rb1vii | 103.6 (3) |
| O1xiii—Rb1—O3xiii | 42.6 (2) | S—O2—Ca2xxvii | 131.0 (6) |
| O1xiv—Rb1—O3xiii | 98.8 (3) | S—O2—Rb2 | 118.2 (5) |
| O3xiv—Rb1—O3xiii | 114.23 (15) | Ca2xxvii—O2—Rb2 | 98.8 (3) |
| O3xii—Rb1—O3xiii | 114.23 (14) | S—O2—Rb1v | 78.8 (4) |
| O4xix—Rb1—O3xxii | 54.3 (3) | Ca2xxvii—O2—Rb1v | 128.1 (4) |
| O4xx—Rb1—O3xxii | 42.4 (2) | Rb2—O2—Rb1v | 99.1 (3) |
| O4xxi—Rb1—O3xxii | 107.7 (3) | Sxxxii—O3—Ca1xviii | 156.2 (7) |
| O1xii—Rb1—O3xxii | 131.6 (2) | Sxxxii—O3—Rb1vii | 89.2 (5) |
| O1xiii—Rb1—O3xxii | 145.4 (2) | Ca1xviii—O3—Rb1vii | 91.9 (4) |
| O1xiv—Rb1—O3xxii | 96.7 (2) | Sxxxii—O3—Rb2xv | 87.6 (5) |
| O3xiv—Rb1—O3xxii | 58.5 (4) | Ca1xviii—O3—Rb2xv | 115.7 (4) |
| O3xii—Rb1—O3xxii | 104.18 (3) | Rb1vii—O3—Rb2xv | 98.5 (3) |
| O3xiii—Rb1—O3xxii | 138.68 (19) | Sxxxii—O3—Rb1xxxiii | 94.4 (5) |
| O4xix—Rb1—O3xxiii | 42.4 (2) | Ca1xviii—O3—Rb1xxxiii | 86.4 (3) |
| O4xx—Rb1—O3xxiii | 107.7 (3) | Rb1vii—O3—Rb1xxxiii | 174.7 (3) |
| O4xxi—Rb1—O3xxiii | 54.3 (3) | Rb2xv—O3—Rb1xxxiii | 77.7 (3) |
| O1xii—Rb1—O3xxiii | 145.4 (2) | S—O4—Ca1 | 146.3 (7) |
| O1xiii—Rb1—O3xxiii | 96.7 (2) | S—O4—Rb1viii | 113.7 (6) |
| O1xiv—Rb1—O3xxiii | 131.6 (2) | Ca1—O4—Rb1viii | 97.9 (3) |
| O3xiv—Rb1—O3xxiii | 104.18 (3) |
| Symmetry codes: (i) z, x, y; (ii) y, z, x; (iii) z−1/2, −x+1/2, −y+1; (iv) −x+1/2, −y+1, z−1/2; (v) −y+1, z−1/2, −x+1/2; (vi) −y+3/2, −z+1, x−1/2; (vii) −y+1, z−1/2, −x+3/2; (viii) z−1/2, −x+3/2, −y+1; (ix) −y, z+1/2, −x+1/2; (x) −y+1/2, −z, x+1/2; (xi) z+1/2, −x+1/2, −y; (xii) −x+1, y+1/2, −z+3/2; (xiii) y+1/2, −z+3/2, −x+1; (xiv) −z+3/2, −x+1, y+1/2; (xv) z+1/2, −x+1/2, −y+1; (xvi) −x+1/2, −y+1, z+1/2; (xvii) −y+1, z+1/2, −x+1/2; (xviii) −y+1/2, −z+1, x+1/2; (xix) −z+1, x+1/2, −y+3/2; (xx) x+1/2, −y+3/2, −z+1; (xxi) −y+3/2, −z+1, x+1/2; (xxii) x+1/2, −y+3/2, −z+2; (xxiii) −z+2, x+1/2, −y+3/2; (xxiv) −y+3/2, −z+2, x+1/2; (xxv) −z+1, x−1/2, −y+1/2; (xxvi) x−1/2, −y+1/2, −z+1; (xxvii) −y+1/2, −z+1, x−1/2; (xxviii) −z, x−1/2, −y+1/2; (xxix) x−1/2, −y+1/2, −z; (xxx) −y+1/2, −z, x−1/2; (xxxi) x, y, z−1; (xxxii) x, y, z+1; (xxxiii) z−1/2, −x+3/2, −y+2. |
| Ca1—O4 | 2.284 (10) | Rb2—O2 | 3.018 (10) |
| Ca1—O3i | 2.299 (10) | Rb2—O1vii | 3.118 (10) |
| Ca2—O1ii | 2.304 (9) | Rb2—O3vii | 3.338 (11) |
| Ca2—O2iii | 2.338 (9) | S—O1viii | 1.449 (9) |
| Rb1—O4iv | 3.028 (11) | S—O2 | 1.447 (9) |
| Rb1—O1v | 3.224 (9) | S—O3viii | 1.456 (10) |
| Rb1—O3v | 3.239 (12) | S—O4 | 1.466 (10) |
| Rb1—O3vi | 3.464 (13) |
| Symmetry codes: (i) z−1/2, −x+1/2, −y+1; (ii) −z+3/2, −x+1, y+1/2; (iii) −y+1, z+1/2, −x+1/2; (iv) −z+1, x+1/2, −y+3/2; (v) −x+1, y+1/2, −z+3/2; (vi) −z+2, x+1/2, −y+3/2; (vii) −z+1, x−1/2, −y+1/2; (viii) x, y, z−1. |
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The double sulfate salts with formula A2B2(SO4)3 adopting the langbeinite-type structure have attracted great interest due to their ferroelastic or ferroelectric properties and first-order phase transitions (Ukeda et al., 1995; Dilanian et al., 1999 and references therein). Numerous compounds with A = NH4, K, Rb, Tl, Cs and B = Mn, Ca, Mg, Fe, Co, Ni, Zn, Cd (e.g. Zemann & Zemann, 1957; Guelylah et al., 1996; Guelylah & Madariaga, 2003) have been characterized up to now. Gattow and Zemann (1957) mentioned the possible synthesis of 26 double sulfates, including large monovalent cations. Notable differences exist between langbeinite-type and Nasicon-type structures (Sizova et al., 1981; Droß & Glaum, 2004). In the Nasicon-type structures, four interstitial vacant sites are present, while langbeinite-type structures have only two.
A projection of the crystal structure of langbeinite-type Rb2Ca2(SO4)3 is given in Fig. 1. It is characterized by the presence of alternating SO4 tetrahedra and CaO6 octahedra, linked by sharing corners, to establish a [Ca2(SO4)3]2− framework. The two independent Rb+ ions are located in the voids of this arrangement.
The SO4 tetrahedra are quite regular, with an average S—O distance of 1.455 Å, which is virtually the same as that observed in the isotypic Rb2Cd2(SO4)2 (1.455 Å; Guelylah & Madariaga, 2003). In the title compound sulfur has a bond valence sum (BVS) of 6.69 valence units (expected 6) as calculated with the values given by Brown & Altermatt (1985). The [Ca1O6] octahedron is quite regular, with dav(Ca1—O) = 2.292 Å, whereas the [Ca2O6] octahedron is considerably distorted, with dav(Ca2—O) = 2.321 Å. Rb1 has twelve oxygen neighbours with dav(Rb1—O) = 3.239 Å and a BVS of 1.03 (expected 1). Rb2 is ninefold coordinated with dav(Rb2—O) = 3.158 Å and a BVS of 0.852 (expected 1).